Movable microstructure with contactless stops

ABSTRACT

A movable microstructure is provided that mitigates stiction. A substrate is provided on which a structural linkage is connected to support a structural film. A hold electrode is connected with the substrate at a position laterally beyond an orthogonal projection of the structural film on the substrate. It is configured to hold the structural film electrostatically in a tilted position with respect to the substrate upon application of a potential difference between the structural film and the hold electrode. Because of its positioning with respect to the structural film, it is ensured that the structural film is not in contact with the substrate when the structural film is being held by the hold electrode.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/705,390 entitled “BISTABLE MIRROR WITH CONTACTLESS STOPS,” filed Nov.10, 2003, U.S. Pat. No. 6,778,304, which is a divisional of U.S. patentapplication Ser. No. 09/899,004, entitled “BISTABLE MIRROR WITHCONTACTLESS STOPS,” filed Jul. 3, 2001, U.S. Pat. No. 6,657,759. Thisapplication is also related to the following U.S. Patents.: U.S. Pat.No. 6,701,037, entitled “MEMS-BASED NONCONTACTING FREE-SPACE OPTICALSWITCH” by Bevan Staple and Richard Roth; U.S. Pat. No. 6,614,581,entitled “METHODS AND APPARATUS FOR PROVIDING A MULTI-STOP MICROMIRROR,”filed Jul. 30, 2001 by David Paul Anderson, and U.S. Pat. No. 6,625,342,entitled “SYSTEMS AND METHODS FOR OVERCOMING STICTION USING A LEVER,”filed Jul. 3, 2001 by Bevan Staple, David Paul Anderson and LilacMuller; all of which are herein incorporated by reference in itsentirety for all purposes.

BACKGROUND OF THE INVENTION

This application relates generally to microelectromechanical systems,and more particularly to MEMS devices and methods configured to avoidstiction.

In recent years, increasing emphasis has been made on the development oftechniques for producing microscopic systems that may be tailored tohave specifically desired electrical and/or mechanical properties. Suchsystems are generically described as microelectromechanical systems(MEMS) and are desirable because they may be constructed withconsiderable versatility despite their very small size. In a variety ofapplications, MEMS component structures may be fabricated to move insuch a fashion that there is a risk of stiction between that componentstructure and some other aspect of the system. One such example of aMEMS component structure is a micromirror, which is generally configuredto reflect light from at least two positions. Such micromirrors findnumerous applications, including as parts of optical switches, displaydevices, and signal modulators, among others.

In many applications, such as may be used in fiber-optics applications,such MEMS-based devices may include hundreds or even thousands ofmicromirrors arranged as an array. Within such an array, each of themicromirrors should be accurately aligned with both a target and asource. Such alignment is generally complex and typically involvesfixing the location of the MEMS device relative to a number of sourcesand targets. If any of the micromirrors is not positioned correctly inthe alignment process and/or the MEMS device is moved from the alignedposition, the MEMS device will not function properly.

In part to reduce the complexity of alignment, some MEMS devices providefor individual movement of each of the micromirrors. An example isprovided in FIGS. 1A-1C illustrating a particular MEMS micromirrorstructure that may take three positions. Each micromirror includes areflective surface 116 mounted on a micromirror structural film 112 thatis connected by a structural linkage 108 to an underlying substrate 104.Movement of an individual micromirror is controlled by energizingactuators 124 a and/or 124 b disposed underneath the micromirror onopposite sides of the structural linkage 108. Hard stops 120 a and 120 bare provided to stop the action of the micromirror structural film 112.Energizing the actuator 124 a on the left side of the structural linkage108 causes the micromirror to tilt on the structural linkage 108 towardsthat side until one edge of the micromirror structural film 112 contactsthe left hard stop 120 a, as shown in FIG. 1A. Alternatively, theactuator 124 b on the right side of the structural linkage 108 may beenergized to cause the micromirror to tilt in the opposite direction, asshown in FIG. 1B. When both actuators are de-energized, as shown in FIG.1C, the micromirror returns to a static position horizontal to thestructural linkage 108. In this way, the micromirror may be moved to anyof three positions. This ability to move the micromirror provides adegree of flexibility useful in aligning the MEMS device, although thealignment complexity remains significant. Sometimes hard stops 120 a and120 b are not provided so that the micromirror structural film 112 is indirect contact with the substrate 104.

In certain applications, once the micromirror is moved to the properposition, it may remain in that position for ten years or more. Thus,for example, one side of an individual micromirror structural film mayremain in contact with the hard stop or substrate for extended periods.Maintaining such contact increases the incidence of dormancy-relatedstiction. Such stiction results in the micromirror remaining in a tiltedposition even after the actuators are de-energized. Some theorize thatstiction is a result of molecule and/or charge build up at the junctionbetween the micromirror structural film and the hard stop or substrate.For example, it has been demonstrated that an accumulation of H₂Omolecules at the junction produces capillary forces that increase theincidence of stiction.

Thus, one solution to overcome stiction is to package the MEMS device ina hermetic or inert environment. Such an environment reduces thepossibility of molecule accumulation at the junction. However, suchpackaging is costly and prone to failure where seals break or are notproperly formed. Further, such packaging is incompatible with many typesof MEMS devices. In addition, such packaging does not reduce stictionrelated to charge build up at the junction.

In “Ultrasonic Actuation for MEMS Dormancy-Related Stiction Reduction”,Proceedings of SPIE Vol. 4180 (2000), which is herein incorporated byreference for all purposes, Ville Kaajakari et al. describe a system forovercoming both molecule and charge related stiction. The systemoperates by periodically vibrating an entire MEMS device to overcomestiction forces. While there is evidence that vibrating the entire MEMSdevice can overcome stiction at discrete locations within the device,such vibration causes temporary or even permanent misalignment of thedevice. Thus, freeing an individual micromirror often requiresperformance of a costly alignment procedure. Even where the device isnot permanently misaligned by the vibration, it is temporarilydysfunctional while the vibration is occurring.

Thus, there exists a need in the art for systems and methods forovercoming stiction in MEMS devices without causing misalignment.

SUMMARY OF THE INVENTION

Embodiments of the invention are therefore directed to a microstructurefor steering light that mitigates stiction. A substrate is provided onwhich a structural linkage is connected to support a structural film.The structural film includes a reflective coating. A hold electrode isconnected with the substrate at a position laterally beyond anorthogonal projection of the structural film on the substrate. It isconfigured to hold the structural film electrostatically in a tiltedposition with respect to the substrate upon application of a potentialdifference between the structural film and the hold electrode. Becauseof its positioning with respect to the structural film, it is ensuredthat the structural film is not in contact with the substrate when thestructural film is being held by the hold electrode.

In some embodiments, a snap-in electrode is also provided. The snap-inelectrode is connected with the substrate at a position laterally withinthe orthogonal projection of the structural film on the substrate. It isconfigured to tilt an end of the structural film in a direction towardsthe snap-in electrode upon application of a potential difference betweenthe structural film and the snap-in electrode.

The hold electrode may be configured as a comb structure having aplurality of teeth. With such a configuration, a plurality of tiltedpositions for the structural film may be realized by the application ofvarious potential differences between the structural film and the holdelectrode. For example, it may be configured such that an increase inpotential difference results in a hold position that deviates morestrongly from horizontal.

The microstructure may be configured in different embodiments with acantilever arrangement or with a torsion-beam arrangement. Inembodiments that use the torsion-beam arrangement, a second holdelectrode and/or second snap-in electrode may be provided on an oppositeside of the structural linkage.

Embodiments of the invention are also directed to a method forfabricating a microstructure for steering light. A structural linkage isformed on a substrate. A structural film is formed on the structurallinkage. A reflective coating is deposited on the structural film. Ahold electrode is formed on the substrate at a position laterally beyondan orthogonal projection of the structural film on the substrate andconfigured to hold the structural film electrostatically in a tiltedposition with respect to the substrate upon application of a potentialdifference between the structural film and the hold electrode. A snap-inelectrode may additionally be formed to tilt the end of the structuralfilm towards the snap-in electrode upon application of a potentialdifference between the structural film and the snap-in electrode. Thehold electrode may be fabricated as a comb structure to permit theselection of a plurality of tilted positions with variation in thepotential difference applied. The microstructure may also be fabricatedwith cantilever or torsion-beam configurations. For embodimentsfabricated according to torsion-beam configurations, additional holdand/or snap-in electrodes may be formed on the substrate opposite thestructural linkage.

Further embodiments provide a method for operating an optical switch. Afirst end of a micromirror assembly is tilted towards a substrate byapplying a first electrostatic force. Thereafter, the micromirrorassembly is held in a first tilted position with respect to thesubstrate with a second electrostatic force originating from a pointlaterally beyond an orthogonal projection of the micromirror assembly onthe substrate. In one embodiment, the micromirror assembly is releasedfrom the first tilted position. Thereafter, a second end of themicromirror assembly is tilted towards the substrate by applying a thirdelectrostatic force. Thereafter, the micromirror assembly is held in asecond tilted position with respect to the substrate with a fourthelectrostatic force that originates from a point laterally beyond theorthogonal projection of the micromirror assembly on the substrate. In acertain embodiment, the first tilted position is selected from aplurality of possible first tilted positions by establishing a potentialdifference between the micromirror assembly and a first electrode usedto establish the second electrostatic force, and the second tiltedposition is selected from a plurality of possible second tiltedpositions by establishing a potential difference between the micromirrorassembly and a second electrode used to establish the fourthelectrostatic force.

In still other embodiments, a wavelength router is provided thatincorporates a microstructure for steering light. The wavelength routeris configured for receiving light having a plurality of spectral bandsat an input port and for directing subsets of the spectral bands to aplurality of output ports. A free-space optical train is disposedbetween the input port and the output ports providing optical paths forrouting the spectral bands. The optical train also includes a dispersiveelement disposed to intercept light traveling from the input port. Arouting mechanism is provided having at least one dynamicallyconfigurable routing element to direct a given spectral band todifferent output ports. The dynamically configurable routing elementincludes a micromirror assembly connected with a substrate by astructural linkage. A hold electrode connected with the substrate at aposition laterally beyond an orthogonal projection of the micromirrorassembly on the substrate is configured to hold the micromirror assemblyelectrostatically in a first tilted position with respect to thesubstrate upon application of a potential difference between themicromirror assembly and the hold electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings wherein like reference numerals are usedthroughout the several drawings to refer to similar components. In someinstances, a sublabel is associated with a reference numeral and isenclosed in parentheses to denote one of multiple similar components.When reference is made to a reference numeral without specification toan existing sublabel, it is intended to refer to all such multiplesimilar components.

FIGS. 1A, 1B, and 1C are cross-sectional drawings of a tiltingmicromirror in three positions effected by actuation of differentactuators;

FIGS. 2A, 2B, 2C, 2D, and 2E are cross-sectional drawings of atorsion-beam micromirror configuration in accordance with the invention;

FIG. 3 is a schematic drawing defining a geometry of anelectromechanical system defined by the torsion-beam micromirrorassembly;

FIG. 4 is a graph illustrating the behavior of capacitive energy storedin one micromirror configuration in accordance with the invention;

FIGS. 5A, 5B, and 5C are cross-sectional drawings of a cantilevermicromirror configuration in accordance with the invention;

FIGS. 6A, 6B, 6C, and 6D are cross-sectional drawings of a multistablemicromirror configuration in accordance with the invention;

FIG. 6E is a graph illustrating the behavior of capacitive energy storedin a multistable micromirror configuration;

FIGS. 7A, 7B, and 7C are schematic top, side, and end views,respectively, of one embodiment of a wavelength router that usesspherical focusing elements;

FIGS. 8A and 8B are schematic top and side views, respectively, of asecond embodiment of a wavelength router that uses spherical focusingelements; and

FIG. 9 is a schematic, top view of a third embodiment of a wavelengthrouter that uses spherical focusing elements; and

FIGS. 10A and 10B are side and top views of an implementation of amicromirror retroreflector array.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

1. Introduction

Embodiments of the invention are directed to MEMS methods and devices inwhich a microstructure is held in one of at least two possible stablepositions without contacting either a substrate or hard stop. In certainembodiments, the microstructure is a micromirror that may be rotated toat least two such positions. Because of the ready applicability of sucha rotating micromirror to optical-switch applications, some of theembodiments are directed to a wavelength router that uses opticalswitching. The stability of the microstructure positions is achievedwithout contact by employing electrostatic fields to hold themicrostructure. Since there is no direct contact with themicrostructure, stiction is thereby avoided. As will be clear to thoseof skill in the art from the following description, the invention may beadapted to different types of micromirror configurations, includingcantilever micromirrors and torsion-beam micromirrors.

It is noted that throughout herein micromirror configurations are shownschematically in the figures for illustrative purposes. As will beunderstood by those of skill in the art, the point of rotation of themicromirror structural film should be selected so that in the desiredstatic micromirror configurations both the forces on the structural filmand the torques about the point of rotation cancel.

2. Torsion-beam Micromirror

One embodiment of the invention as applied to a torsion-beam micromirrorconfiguration is illustrated in FIGS. 2A-2E. Each micromirror includes areflective surface 216 mounted on a micromirror structural film 212 thatis connected by at least one structural linkage 208 to an underlyingsubstrate 204. In some embodiments, multiple structural linkages 208 areprovided in the plane orthogonal to the page, the axis of rotation ofthe micromirror structural film 212 being defined by the alignment ofthe structural linkages. In one such embodiment, two structural linkages208 are provided approximately on opposite sides of the micromirroralong the axis of rotation. Two snap-in electrodes 224 a and 224 b andtwo hold electrodes 220 a and 220 b are provided on the substrate 204,with one of each type of electrode provided on either side of thestructural linkage 208. The electrodes 220 and 224 and structural film212 may be fabricated using standard MEMS techniques. Such MEMStechniques typically involve a combination of depositing structuralmaterial, such as polycrystalline silicon, depositing sacrificialmaterial, such as silicon oxide, and dissolving the sacrificial materialduring a release step, for example with hydrofluoric acid (HF). It isthus sometimes convenient to identify the different structural layers ina MEMS microstructure as “poly-N” layers, where N denotes that aparticular such layer was the Nth polysilicon layer deposited in aprocess that included multiple depositions. Often the first such layeris described as the “poly-0” layer.

The hold electrodes 220 a and 220 b are connected with the substrate 204at a position laterally beyond an orthogonal projection of thestructural film 212 onto the substrate 204. With such a configuration,the hold electrodes 220 a and 220 b are outside the region underneaththe micromirror structural film 212. This geometry ensures that when themicromirror is in the hold positions shown in FIGS. 2C and 2E, themicromirror structural film 212 is not in contact with the substrate204. In certain embodiments, the hold electrodes 220 a and 220 b have agreater height above the substrate 204 than the snap-in electrodes 224 aand 224 b. The electrodes may thus be fabricated with MEMS techniques inwhich a poly-0 layer is deposited to form the structure of the snap-inelectrodes 224 a and 224 b and the lower part of the structure of thehold electrodes 222 a and 222 b. The remainder of the structure of thehold electrodes 221 a and 221 b may be fabricated with a subsequentlydeposited poly-1 layer. The micromirror structural film 212 is formedwith a poly-3 layer. The reflective surface 216 is formed by depositinga layer of reflective metal, such as gold.

FIG. 2A shows the static horizontal configuration of the micromirrorwhen all four of the electrodes 220 a, 220 b, 224 a, and 224 b arecommonly grounded with the structural linkage 208. According toembodiments of the invention, the micromirror may be deflected to aposition tilted to the right, as shown in FIG. 2C, or to a positiontilted to the left, as shown in FIG. 2E. Either of these tiltedpositions is maintained through activation of the right or left holdelectrode 220 as appropriate, such that the micromirror and structuralfilm 212 have no contact with the substrate 204 or with a hard stop.

In order to achieve the right-tilted position, for example, the rightsnap-in electrode 224 b is activated, as shown in FIG. 2B, by applying avoltage V to that electrode with respect to the common ground. Thepotential difference between the structural film 212 and the rightsnap-in electrode 224 b thus creates an electric field with dotted fieldlines 228 shown. That right side of the structural film is thusdeflected downwards such that the structural film 216 may come intocontact with the substrate 204. Subsequently, the right hold electrode220 b is activated and the right snap-in electrode 224 b is deactivated,creating an electric field between the structural film 212 and the righthold electrode 220 b, shown by dotted electric field lines 232. Thiselectric field thus maintains the micromirror in its tilted positionwithout any contact with the substrate 204 or a hard stop, therebyavoiding stiction problems.

The micromirror may similarly be tilted to the left position shown inFIG. 2E. Activation of the left snap-in electrode 224 a deflects thestructural film 212 to the left, perhaps in contact with the substrate204, with the electric field shown by dotted electric field lines 236.Subsequent deactivation of the left snap-in electrode 224 a andactivation of the left hold electrode 220 a creates an electric fieldshown by dotted field lines 238 that acts to hold the micromirror in itsleft tilted position without contact with the substrate 204 or with ahard stop.

The electromechanical behavior of the system may be better understoodwith reference to FIGS. 3 and 4. In FIG. 3, the geometry is shown for anarrangement in which a micromirror structural film 312 is held in aleft-tilted position on a structural linkage 308 supported by asubstrate 304. The structural linkage point O may be defined as anorigin for the system with vectors r defining spatial positions, withangle θ defining the tilt. The electric field E that acts to hold themicromirror structural film 312 in position may be approximatelyrepresented with image charges P and Q. The potential difference createdby activation of the hold electrode creates a capacitive arrangementdefined by the micromirror structural film 312, the active electrode,and the gap between them. This capacitive arrangement has a capacitance${C = \frac{2\quad U}{V^{2}}},$where U is the capacitive energy stored and V is the potentialdifference applied to the electrode. The capacitive energy may bedefined in terms of the displacement and electric fields as$U = {\frac{1}{2}{\int{{\mathbb{d}r}\quad{{D(r)} \cdot {{E(r)}.}}}}}$The displacement field D(r) is related to the electric field E(r)according to the permittivity ∈(r) of the air in the gap,D(r)=∈E(r)

FIG. 4 illustrates the approximate dependence of the capacitive energy Uas a function of the tilt angle θ. In orienting the micromirrorstructural film relative to the active holding electrode, the systemwill seek to minimize the energy U by selecting angle θ₀. The fact thatthe system has a preferred tilt angle θ₀ may alternatively be understoodfrom the fact that the attractive electrostatic force is inverselyproportional to the square of the separation between the electrode andthe micromirror structural film; the system thus seeks to minimize thatseparation. In some embodiments, it is preferable to activate the holdelectrode when the system is already oriented near θ₀. This is achievedin such embodiments, as illustrated in FIGS. 2B and 2D, by using one ofthe snap-in electrodes to move the micromirror structural film such thatθ≅θ₀ before activation of the hold electrode.

3. Cantilever Micromirror

Embodiments of the invention may also be used with cantilevermicromirror arrangements. Cantilever arrangements are similar totorsion-beam arrangements, but use a flexure positioned at one side ofthe micromirror. An example of a cantilever micromirror arrangement inaccordance with the invention is illustrated in FIGS. 5A-5C. Thecantilever arrangement generally permits a static horizontal position,as shown in FIG. 5A, and a tilted position, as shown in FIG. 5C. Likethe torsion-beam arrangement, the tilted position of the cantileverarrangement is maintained without contacting either the substrate 504 ora hard stop.

Each micromirror includes a reflective surface 516 mounted on amicromirror structural film 512 that is connected by at least oneflexure 508 to an underlying substrate 504. A snap-in electrode 524 anda hold electrode 520 are provided. The hold electrode 520 may becomposed of a poly-0 layer 522 and a poly-1 layer 521. When the snap-inelectrode 520 and hold electrode 525 are both commonly grounded with theflexure 508, as shown in FIG. 5A, the micromirror is in the horizontalposition. The tilted position may be reached by activating the snap-inelectrode 524 to produce the electric field shown with electric fieldlines 528 in FIG. 5B, and thereby move the micromirror structural film512 downwards, such that it may come in contact with the substrate.Subsequent deactivation of the snap-in electrode 524 and activation ofthe hold electrode 520 causes the electric field shown with electricfield lines 532 in FIG. 5C to hold the micromirror structural film 512in its tilted position in a contactless fashion. As for the torsion-beamconfiguration, the hold electrode 520 is connected with the substrate504 at a position laterally beyond an orthogonal projection of thestructural film 12 onto the substrate 504. With such a configuration,the hold electrode 520 is outside the region underneath the micromirrorstructural film 512. This geometry ensures that when the micromirror isin the hold position shown in FIGS. 5C, the micromirror structural film512 is not in contact with the substrate 504.

4. Multistable Micromirror Configurations

FIGS. 6A-6D illustrates an embodiment of the invention in whichmultistable micromirror configurations are realized. The illustration inFIGS. 6A-6D is shown for a cantilever-type micromirror assembly,although it may be adapted to other micromirror configurations,including torsion-beam configurations. In FIGS. 6A-6D, a reflectivesurface 616 is mounted on a micromirror structural film 612 connected tounderlying substrate 604 by at least one flexure 608. A snap-inelectrode 624 may be provided such that the creation of a potentialdifference between it and the micromirror structural film 612 may beused to tilt the micromirror, perhaps in contact with the substrate 604as described with respect to FIG. 5B. The hold electrode 620 isconfigured as a comb structure with multiple teeth 621, 622, and 623.The teeth are configured at different heights above the substrate andmay be used to achieve different tilt orientations of the micromirror.As in the previous embodiments, the hold electrode 620 is connected withthe substrate 604 at a position laterally beyond an orthogonalprojection of the structural film 612 onto the substrate 604. With sucha configuration, the hold electrode 620 is outside the region underneaththe micromirror structural film 612. Thus, as illustrated in FIGS.6B-6D, the application of a potential difference between the holdelectrode 620 and the micromirror structural film 612 results in anelectric field that holds the micromirror in a tilted position withoutcontact with the substrate 604 or a hard stop.

The degree of tilt is dependent on the size of the potential difference,as may be understood with further reference to FIG. 6E. For example,when a potential difference V₁ is applied, as shown in FIG. 6B, acapacitive system is formed between the micromirror structural film 612and the hold electrode 620. The resulting electric field is shown withfield lines 628 and the energy behavior U as a function of tilt angle θis shown in FIG. 6E. The general behavior of U as a function of θ issimilar to that described with respect to FIG. 4, with the systemseeking the energy minimum, and thereby being held at tilt angle θ=θ₁.For a smaller potential difference V₂<V₁, the energy behavior U has thesame qualitative behavior, but has a shallower minimum located at ahigher tilt angle, as shown in FIGS. 6C and 6E. Thus, upon applicationof potential difference V₂, the system seeks a hold position at tiltangle θ=θ₂, with the electric field shown by field lines 630. Similarly,a still smaller potential difference V₃<V₂ results in an energy curve inFIG. 6E having a still shallower minimum at a still higher angle.Accordingly, the system seeks a hold position at tilt angle θ =θ₃ withelectric field lines 632, as shown in FIG. 6D. The snap-in electrode 624is useful for achieving an initial tilt for the micromirror structuralfilm 612.

5. Fiber-Optics Applications

a. Wavelength Router

Tilting micromirrors according to the embodiments described above, andtheir equivalents, may be used in numerous applications as parts ofoptical switches, display devices, or signal modulators, among others.One particular application of such tilting micromirrors is as opticalswitches in a wavelength router such as may be used in fiber-optictelecommunications systems. One such wavelength router is described indetail in U.S. Pat. No. 6,501,877, entitled “Wavelength Router,” whichis herein incorporated by reference in its entirety, including theAppendix, for all purposes. The various micromirror embodiments may beused in that wavelength router or may be incorporated into otherwavelength routers as optical switches where it is desirable to avoidstiction problems.

Fiber optic telecommunications systems are currently deploying arelatively new technology called dense wavelength division multiplexing(DWDM) to expand the capacity of new and existing optical fiber systemsto help satisfy the steadily increasing global demand for bandwidth. InDWDM, multiple wavelengths of light simultaneously transport informationthrough a single optical fiber. Each wavelength operates as anindividual channel carrying a stream of data. The carrying capacity or afiber is multiplied by the number of DWDM channels used. Today DWDMsystems employing up to 80 channels are available from multiplemanufacturers, with more promised in the future.

In all telecommunication networks, there is the need to connectindividual channels (or circuits) to individual destination points, suchas an end customer or to another network. Systems that perform thesefunctions are called cross-connects. Additionally, there is the need toadd or drop particular channels at an intermediate point. Systems thatperform these functions are called add-drop multiplexers (ADMs). All ofthese networking functions are performed with a wavelength router usedwith the current invention by an all-optical network. Optical networksdesigned to operate at the wavelength level are commonly called“wavelength routing networks” or “optical transport networks” (OTN). Ina wavelength routing network, the individual wavelengths in a DWDM fibermust be manageable. New types of photonic network elements operating atthe wavelength level are required to perform the cross-connect, ADM andother network switching functions. Two of the primary functions areoptical add-drop multiplexers (OADM) and wavelength-selectivecross-connects (WSXC).

Wavelength routing functions may be performed optically with afree-space optical train disposed between the input ports and the outputports, and a routing mechanism. The free-space optical train can includeair-spaced elements or can be of generally monolithic construction. Theoptical train includes a dispersive element such as a diffractiongrating, and is configured so that the light from the input portencounters the dispersive element twice before reaching any of theoutput ports. The routing mechanism includes one or more routingelements and cooperates with the other elements in the optical train toprovide optical paths that couple desired subsets of the spectral bandsto desired output ports. The routing elements are disposed to interceptthe different spectral bands after they have been spatially separated bytheir first encounter with the dispersive element.

FIGS. 7A, 7B, and 7C are schematic top, side, and end views,respectively, of one embodiment of a wavelength router 10. Its generalfunctionality is to accept tight having a plurality N of spectral bandsat an input port 12, and to direct subsets of the spectral bands todesired ones of a plurality M of output ports, designated 15(l) . . .15(M). The output ports are shown in the end view of FIG. 7C as disposedalong a line 17 that extends generally perpendicular to the top view ofFIG. 7A. Light entering the wavelength router 10 from input port 12forms a diverting beam 18, which includes the different spectral bands.Beam 18 encounters a lens 20 that collimates the light and directs it toa reflective diffraction grating 25. The grating 25 disperses the lightso that collimated beams at different wavelengths are directed atdifferent angles back towards the lens 20.

Two such beams are shown explicitly and denoted 26 and 26′, the latterdrawn in dashed lines. Since these collimated beams encounter the lens20 at different angles, they are focused towards different points alonga line 27 in a transverse plane extending in the plane of the top viewof FIG. 7A. The focused beams encounter respective ones of a pluralityof retroreflectors that may be configured according as contactlessmicromirror optical switches as described above, designated 30(l) . . .30(N), located near the transverse plane. The beams are directed back,as diverging beams, to the lens 20 where they are collimated, anddirected again to the grating 25. On the second encounter with thegrating 25, the angular separation between the different beams isremoved and they are directed back to the lens 20, which focuses them.The retroreflectors 30 may be configured to send their intercepted beamsalong a reverse path displaced along respective lines 35(l) . . . 35(N)that extend generally parallel to line 17 in the plane of the side viewof FIG. 7B and the end view of FIG. 2C, thereby directing each beam toone or another of output ports 15.

Another embodiment of a wavelength router, designated 10′, isillustrated with schematic top and side views in FIGS. 8A and 8B,respectively. This embodiment may be considered an unfolded version ofthe embodiment of FIGS. 7A-7C. Light entering the wavelength router 10′from input port 12 forms diverging beam 18, which includes the differentspectral bands. Beam 18 encounters a first lens 20 a, which collimatesthe light and directs it to a transmissive grating 25′. The grating 25′disperses the light so that collimated beams at different wavelengthsencounter a second lens 20 b, which focuses the beams. The focused beamsare reflected by respective ones of plurality of retroreflectors 30,which may also be configured as contactless micromirror opticalswitches, as diverging beams, back to lens 20 b, which collimates themand directs them to grating 25′. On the second encounter, the grating25′ removes the angular separation between the different beams, whichare then focused in the plane of output ports 15 by lens 20 a.

A third embodiment of a wavelength router, designated 10″, isillustrated with the schematic top view shown in FIG. 9. This embodimentis a further folded version of the embodiment of FIGS. 7A-7C, shown as asolid glass embodiment that uses a concave reflector 40 in place of lens20 of FIGS. 7A-7C or lenses 20 a and 20 b of FIGS. 8A-8B. Light enteringthe wavelength router 10″ from input port 12 forms diverging beam 13,which includes the different spectral bands. Beam 18 encounters concavereflector 40, which collimates the light and directs it to reflectivediffraction grating 25, where it is dispersed so that collimated beamsat different wavelengths are directed at different angles back towardsconcave reflector 40. Two such beams are shown explicitly, one in solidlines and one in dashed lines. The beams then encounter retroreflectors30 and proceed on a return path, encountering concave reflector 40,reflective grating 25′, and concave reflector 40, the final encounterwith which focuses the beams to the desired output ports. Again, theretroreflectors 30 may be configured as contactless micromirror opticalswitches.

b. Contactless-Micromirror Optical-Switch Retroreflector Implementations

FIG. 10A shows schematically the operation of a retroreflector,designated 30 a, that uses contactless-micromirror optical switches.FIG. 10B is a top view. A pair of micromirror arrays 62 and 63 ismounted to the sloped faces of a V-block 64. A single micromirror 65 inmicromirror array 62 and a row of micromirrors 66(l . . . M) inmicromirror array 63 define a single retroreflector. Micromirror arraysmay conveniently be referred to as the input and output micromirrorarrays, with the understanding that light paths are reversible. The leftportion of the figure shows micromirror 65 in a first orientation so asto direct the incoming beam to micromirror 66(l), which is oriented 90°with respect to micromirror 65's first orientation to direct the beamback in a direction opposite to the incident direction. The right halfof the figure shows micromirror 65 in a second orientation so as todirect the incident beam to micromirror 66(M). Thus, micromirror 65 ismoved to select the output position of the beam, while micromirrors 66(l. . . M) are fixed during normal operation. Micromirror 65 and the rowof micromirrors 66(l . . . M) can be replicated and displaced in adirection perpendicular to the plane of the figure. While micromirrorarray 62 need only be one-dimensional, it may be convenient to provideadditional micromirrors to provide additional flexibility.

In one embodiment, the micromirror arrays are planar and the V-groovehas a dihedral angle of approximately 90° so that the two micromirrorarrays face each other at 90°. This angle may be varied for a variety ofpurposes by a considerable amount, but an angle of 90° facilitatesrouting the incident beam with relatively small angular displacements ofthe micromirrors. In certain embodiments, the input micromirror arrayhas at least as many rows of micromirrors as there are input ports (ifthere are more than one), and as many columns of mirrors as there arewavelengths that are to be selectably directed toward the outputmicromirror array. Similarly, in some embodiments, the outputmicromirror array has at least as many rows of micromirrors as there areoutput ports, and as many columns of mirrors as there are wavelengthsthat are to be selectably directed to the output ports.

In a system with a magnification factor of one-to-one, the rows ofmicromirrors in the input array are parallel to each other and thecomponent of the spacing from each other along an axis transverse to theincident beam corresponds to the spacing of the input ports. Similarly,the rows of micromirrors in the output array are parallel to each otherand spaced from each other (transversely) by a spacing corresponding tothat between the output ports. In a system with a differentmagnification, the spacing between the rows of mirrors would be adjustedaccordingly.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Accordingly, the above description should not be taken aslimiting the scope of the invention, which is defined in the followingclaims.

1. A microstructure comprising: a substrate; a structural linkageconnected with the substrate and supporting a structural film; and afirst hold electrode connected with the substrate at a positionlaterally beyond an orthogonal projection of the structural film on thesubstrate and configured to hold the structural film electrostaticallyin a first tilted position with respect to the substrate uponapplication of a potential difference between the structural film andthe first hold electrode.
 2. The microstructure recited in claim 1further comprising a first snap-in electrode connected with thesubstrate at a position laterally within the orthogonal projection ofthe structural film on the substrate and configured to tilt an end ofthe structural film in a direction towards the first snap-in electrodeupon application of a potential difference between the structural filmand the first snap-in electrode.
 3. The microstructure recited in claim2 wherein the first snap-in electrode comprises a polysilicon layer. 4.The microstructure recited in claim 3 wherein the first hold electrodecomprises a polysilicon bilayer.
 5. The microstructure recited in claim1 wherein the first hold electrode comprises a comb structure having aplurality of teeth, the first hold electrode being configured such thatthe first tilted position is defined by an angle with respect to thesubstrate that depends on the potential difference between thestructural film and the first hold electrode.
 6. The microstructurerecited in claim 5 wherein the angle of the first tilted positiondeviates increasingly from horizontal with an increase in the potentialdifference between the structural film and the first electrode.
 7. Themicrostructure recited in claim 1 further comprising a second holdelectrode connected with the substrate at a position laterally beyond anorthogonal projection of the structural film and the substrate and on anopposite side of the structural linkage from the first hold electrode,wherein the second hold electrode is configured to hold the structuralfilm electrostatically in a second tilted position with respect to thesubstrate upon application of a potential difference between thestructural film and the second hold electrode.
 8. The microstructurerecited in claim 7 further comprising first and second snap-inelectrodes connected with the substrate at positions laterally withinthe orthogonal projection of the structural film on the substrate and onopposite sides of the structural linkage, each of the first and secondsnap-in electrodes being configured to tilt an end of the structuralfilm in a direction towards that snap-in electrode upon application of apotential difference between the structural film and that snap-inelectrode.
 9. The microstructure recited in claim 8 wherein the firstand second snap-in electrodes comprise a polysilicon layer.
 10. Themicrostructure recited in claim 9 wherein the first and second holdelectrodes comprise a polysilicon bilayer.
 11. The microstructurerecited in claim 7, wherein the first hold electrode comprises a firstcomb structure having a plurality of teeth, the first hold electrodebeing configured such that the first tilted position is defined by afirst angle with respect to the substrate that depends on the potentialdifference between the structural film and the first electrode; andwherein the second hold electrode comprises a second comb structurehaving a plurality of teeth, the second hold electrode being configuredsuch that the second tilted position is defined by a second angle withrespect to the substrate that depends on the potential differencebetween the structural film and the second electrode.
 12. Themicrostructure recited in claim 11, wherein the first angle deviatesincreasingly from horizontal with an increase in the potentialdifference between the structural film and the first electrode; andwherein the second angle deviates increasingly from horizontal with anincrease in the potential difference between the structural film and thesecond electrode.
 13. A method for fabricating a microstructure, themethod comprising: forming a first hold electrode on a substrate;forming a structural linkage on the substrate; and forming a structuralfilm on the structural linkage; wherein the first hold electrode is at aposition laterally beyond an orthogonal projection of the structuralfilm on the substrate and configured to hold the structural filmelectrostatically in a first tilted position with respect to thesubstrate upon application of a potential difference between thestructural film and the first hold electrode.
 14. The method recited inclaim 13 further comprising forming a first snap-in electrode on thesubstrate at a position laterally within the orthogonal projection ofthe structural film and the substrate and configured to tilt an end ofthe structural film in a direction towards the first snap-in electrodeupon application of a potential difference between the structural filmand the first snap-in electrode.
 15. The method recited in claim 13wherein forming a first hold electrode comprises forming a combstructure having a plurality of teeth, wherein the first hold electrodeis configured such that the first tilted position is defined by an anglewith respect to the substrate that depends on the potential differencebetween the structural film and the first hold electrode.
 16. The methodrecited in claim 13 further comprising forming a second hold electrodeon the substrate at a position laterally beyond an orthogonal projectionof the structural film on the substrate and on an opposite side of thestructural linkage from the first hold electrode, wherein the secondhold electrode is configured to hold the structural filmelectrostatically in a second tilted position with respect to thesubstrate upon application of a potential difference between thestructural film and the second hold electrode.
 17. The method recited inclaim 16 further comprising forming first and second snap-in electrodeson the substrate at positions laterally within the orthogonal projectionof the structural film on the substrate and on opposite sides of thestructural linkage, each of the first and second snap-in electrodesbeing configured to tilt an end of the structural film in a directiontowards that snap-in electrode upon application of a potentialdifference between the structural film and that snap-in electrode. 18.The method recited in claim 16 wherein, forming a first hold electrodecomprises forming a first comb structure having a plurality of teeth,wherein the first hold electrode is configured such that the firsttilted position is defined by an angle with respect to the substratethat depends on the potential difference between the structural film andthe first hold electrode; and forming a second hold electrode comprisesforming a second comb structure having a plurality of teeth, wherein thesecond hold electrode is configured such that the second tilted positionis defined by an angle with respect to the substrate that depends on thepotential difference between the structural film and the second holdelectrode.
 19. A method for operating a microstructure having astructural film formed above a substrate, the method comprising: tiltinga first end of the structural film towards a substrate by applying afirst electrostatic force; and thereafter, holding the structural filmin a first tilted position with respect to the substrate with a secondelectrostatic force originating from a point laterally beyond anorthogonal projection of the structural film on the substrate.
 20. Themethod recited in claim 19 further comprising: releasing the structuralfilm from the first tilted position; thereafter, tilting a second end ofthe structural film towards the substrate by applying a thirdelectrostatic force; and thereafter, holding the structural film in asecond tilted position with respect to the substrate with a fourthelectrostatic force originating from a point laterally beyond theorthogonal projection of the structural film on the substrate.
 21. Themethod recited in claim 20 further comprising: selecting the firsttilted position from a plurality of possible first tilted positions byestablishing a potential difference between the structural film and afirst electrode used to establish the second electrostatic force; andselecting the second tilted position from a plurality of possible secondtilted positions by establishing a potential difference between thestructural film and a second electrode used to establish the fourthelectrostatic force.
 22. The method recited in claim 19 furthercomprising selecting the first tilted position from a plurality ofpossible first tilted positions by establishing a potential differencebetween the structural film and a first electrode used to establish thesecond electrostatic force.
 23. A microstructure comprising: supportmeans; tiltable structural means connected with the support means; andfirst electrostatic-field-generation means for providing anelectrostatic field to hold the tiltable structural means in a tiltedposition with respect to the support means, wherein the firstelectrostatic-field-generation means is connected with the support meansat a position laterally beyond an orthogonal projection of the tiltablestructural means on the support means.
 24. The microstructure recited inclaim 23 further comprising second electrostatic-force-generation meansfor tilting the tiltable structural means, wherein the secondelectrostatic-field-generation means is connected with the support meansat a position laterally within the orthogonal projection of the tiltablestructural means on the support means.
 25. The microstructure recited inclaim 23 wherein the first electrostatic-force-generation means isconfigured for providing a plurality of electrostatic fields to hold thetiltable structural means in a respective plurality of tilted positionsdepending on a state of the first electrostatic-force-generation means.26. The microstructure recited in claim 23 wherein the tiltablestructural means comprises torsion-beam means.
 27. The microstructurerecited in claim 23 wherein the tiltable structural means comprisescantilever means.